1. Field of the Invention
The present invention relates to new powder coating compositions which are solid below 30.degree. C. and liquid above 120.degree. C. and contain A1) lacquer polyisocyanates having an NCO functionality of at least 2.1 and containing free isocyanate groups bound to primary or secondary (cydo)aliphatic carbon atoms, and A2) organic polyhydroxyl compounds, particularly polyhydroxy polyurethanes. The invention also relates to the use of these coating compositions for coating heat-resistant substrates, particularly as clear coating compositions for automobiles.
2. Description of the Prior Art
Combinations of organic polyhydroxyl compounds and blocked polyisocyanates which are solid at room temperature constitute valuable resins for thermally crosslinkable polyurethane powder lacquers (for example, see DE-OS 2,105,777, DE-OS 2,542,191, DE-OS 2,735,497, DE-OS 2,801,126, DE-OS 2,812,252, DE-OS 2,946,085, DE-OS 3,143,060, DE-OS 3,434,881, EP-A-0,009,694, EP-A-0,218,040, EP-A-0,286,799, EP-A-0,403,779, EP-A-0,409,745, EP-A-0,460,963, WO 91/15 532, U.S. Pat. No. 3,857,818 and U.S. Pat. No. 4,375,539). The systems disclosed in these references have the common disadvantage that during thermal crosslinking the compounds used as blocking agents are split off and escape into the environment. For environmental and industrial hygiene reasons, special precautions therefore have to be taken to purify the off-gas and/or to recover the blocking agent.
One attempt to eliminate this major disadvantage is manifested in the use of IPDI powder lacquer hardeners which contain linear uretdione and urethane groups and which are free from blocking agents. These hardeners contain free terminal isocyanate groups, urethane groups or urea groups (EP-A 0,045,994, EP-A-0,045,996 and EP-A-0,045,998), and crosslinking occurs with thermal cleavage of the uretdione groups. However, one disadvantage of these hardeners is that they are strictly linear due to their method of synthesis. This does not enable branching of the lacquer to occur, which is necessary to obtain good solvent-resistance, scratch-resistance and high hardness.
In some instances polyisocyanates having free isocyanate groups have been used as crosslinking agents for polyurethane powder lacquers. For example, EP-A-0,009,694 describes solid hardeners based on partially trimerized IPDI, which contain free isocyanate groups in addition to blocked isocyanate groups. However, in the embodiments described in this document the proportion of blocked isocyanate groups is always greater than the proportion of free isocyanate groups, so that there is no risk of premature crosslinking during storage or during homogeneous mixing with resins containing hydroxyl groups in an extruder at temperatures of about 100.degree. C. to 120.degree. C.
EP-0,023,626-B1 also describes polyisocyanates used as powder lacquer hardeners. These polyisocyanates contain free isocyanate groups in addition to isocyanates blocked with .epsilon.-caprolactam. These products are based on a thin film IPDI trimer which has a low monomer content and which only has a low content of free isocyanate groups and a low functionality (.ltoreq.1), based on the free isocyanate groups. Premature crosslinking with the OH components in the extruder also does not occur with these products.
According to EP-A-0,045,994, polyuretdiones of IPDI which contain free terminal isocyanate groups are used as powder lacquer hardeners. However, the free NCO groups react completely with resins containing hydroxyl groups during homogenization in the extruder at 100.degree. to 130.degree. C. (EP-A-0,045,994, page 20, line 13).
The formulation of powder lacquers which are stable during storage and which exclusively contain free isocyanate groups and hydroxyl groups for crosslinking is not possible according to these prior publications.
The use of unblocked polyisocyanates as powder lacquer hardeners has also been proposed in EP-A-0,071,812. However, according to the teachings of this reference crosslinking occurs via an NCO/epoxide reaction, from which the presence of free hydroxyl groups must be absolutely excluded, since crosslinking and gel formation would otherwise occur during the mixing and extrusion of the components. This reference also fails to describe powder coating compositions which are stable during storage and which contain free isocyanate groups in addition to hydroxyl groups.
EP-A-0,193,828, EP-A-0,224,165 and EP-A-0,254,152 describe special polyisocyanates which are solid at room temperature, contain isocyanurate and/or urethane groups, and which have isocyanate groups bound to tertiary (cyclo)aliphatic carbon atoms, as crosslinking components for polyurethane (PUR) powder lacquers. Due to the low reactivity of the tertiary NCO groups, these polyisocyanates can be mixed in unblocked form with powder resins containing OH groups at temperatures above their melting point, without an unwanted reaction occurring. When stoved at 150.degree. to 220.degree. C., preferably at 170.degree. to 190.degree. C., coating compositions formulated from these polyisocyanates produce highly crosslinked, glossy lacquer films with good hardness, flexibility and solvent-resistance.
DE-OS 2,359,538 describes powder coating compositions containing organic polyisocyanates and polyacrylate or polyester resins having free isocyanate groups and hydroxyl groups, which are produced by mixing the vehicle components in the melt. However the examples are exclusively directed to the use of linear diisocyanates and polyhydroxyl compounds with primary hydroxyl groups. This means that the resins of these powder coating compositions are not reactive mixtures which first react with crosslinking on the substrate, but instead are polyurethanes which have already reacted before they are applied and which are still thermoplastic due to the linearity of the diisocyanates. Crosslinking may be achieved by the simultaneous incorporation of epoxide groups, which may be crosslinked via dicarboxylic acids, as described in Example 1 of this reference.
In one attempt to further reduce solvent emissions from coating media, powder lacquers are currently being considered for a series of other applications, such as the primer coating of automobiles, which have previously been reserved for conventional solvent-containing or aqueous lacquer systems. In this respect there is a particular need for powder coating compositions which may be hardened below 160.degree. C., preferably at about 140.degree. C., without splitting off blocking agents and which result in highly crosslinked, solvent-free, flexible coatings.
The known PUR powder lacquers which contain hardeners without blocking agents described above cannot satisfy these requirements for a stoving temperature of about around 140.degree. C. and for a high crosslink density. The prior art hardeners containing urethane groups and uretdione groups have a functionality which is too low to form highly crosslinked coatings. The hardeners having tertiary bound NCO groups can be converted to a highly branched form; however, their low reactivity does not satisfy the requirements for a low stoving temperature, just like the uretdione hardeners.
An object of the present invention is to provide new powder coating compositions which have a high reactivity and can be hardened, without splitting off blocking agents, to form highly crosslinked, solvent-resistant, flexible coatings.
This object may be achieved with the powder coating compositions described in detail below. The essential resins are a mixture of solid polyisocyanates having free isocyanate groups bound to primary and/or secondary carbon atoms and special polyol components having free OH groups.
The powder coating compositions described in detail below are based on the surprising observation that known branched solid polyisocyanates, which have NCO groups bound to primary and/or secondary carbon atoms, can be used as PUR powder lacquer hardeners, if the polyols used only contain OH groups bonded to secondary and/or tertiary carbon atoms. This fact was completely unexpected since, according to the existing prior art described above, the free primary and/or secondary NCO groups used in PUR lacquers normally react with free OH groups in the extruder during the production of the coating composition.